1. Field of the Invention
The present invention relates to a throwaway insert which is detachably attached to a throwaway cutting tool (hereinafter referred to as a cutting tool), and the present invention also relates to a cutting tool provided with such a throwaway insert.
2. Background Art
As examples of such a throwaway insert and cutting tool, the inventors of the present invention have proposed a throwaway insert and a rotating cutting tool in Japanese Unexamined Patent Application, First Publication No. 2000-141123. In this case, the throwaway insert comprises a first cutting edge (a major cutting edge) having a predetermined length, and a second cutting edge (a corner cutting edge) which extends radially and outwardly from the first cutting edge as viewed from an axis of rotation, and which has a relatively small radius of curvature, and the throwaway insert is adapted to be detachably attached to a cutting tool main body (rotating cutting tool) in such a manner that the throwaway insert projects from the outer periphery of the cutting tool main body while a predetermined cutting angle is applied to the first cutting edge when being attached to the cutting tool main body. The rotating cutting tool enables continuous formation, in a workpiece to be machined, of a plane portion and a wall connected to the plane portion with a predetermined angle by rotating about the axis of rotation with the above-mentioned throwaway insert while translating the axis of rotation. According to such a throwaway insert and a cutting tool, if compared with a cutting tool provided with a circular disc-shaped throwaway insert, it is possible to improve efficiency of cutting because the contact length between the cutting tool and the workpiece, and therefore, cutting resistance depending on the contact length are not significantly changed between the case in which the plane portion is machined and the case in which the wall is machined, and because the rotating speed of the cutting tool when the plane portion is machined need not be set as low as when the wall is machined.
In such conventional throwaway insert and cutting tool, a side edge, which is connected to the second cutting edge, and which is directed radially and outwardly with respect to the cutting tool, is formed so as to incline radially and inwardly with respect to the axis of rotation in order to avoid interference with the wall to be machined, and as a result, there is formed a gap between an end of the side edge and the outermost point of the throwaway insert, which defines the maximum radius of rotation. In the above-mentioned throwaway insert, because the side edge, which is connected to the second cutting edge, is formed in a straight shape, and because the gap between the side edge and the wall gradually decreases as the distance from the second cutting edge decreases even though the side edge is inclined radially and inwardly, cut chips tend to be packed up in the narrow gap when, in particular, cutting is performed while the axis of rotation is directed in the horizontal direction, and as a result, the machined surface (the wall) may be damaged, or the throwaway insert may be chipped off. Moreover, the above-mentioned throwaway insert is substantially formed in a polygonal plate shape, and the throwaway insert is attached to the cutting tool main body in such a manner that a pair of side edges, which include therebetween a corner that diagonally faces to the second cutting edge, are engaged with support surfaces of the cutting tool main body. Because the engagement is achieved by abutting flat surfaces, it is difficult to ensure sufficient rigidity in fixation of the throwaway insert against changes in the direction of cutting load applied to the throwaway insert between when the plane portion is machined and when the wall is machined, and as a result, the throwaway insert may be undesirably shifted.
Based on the above circumstances, an object of the present invention is provide a throwaway insert which enables reliably preventing cut chips from being packed up by ensuring clearance between the throwaway insert and a machined surface, and which enables a stable cutting operation by ensuring fixing rigidity in a state in which the throwaway insert is attached to a cutting tool main body. Another object of the present invention is to provide a cutting tool having such a throwaway insert.
In order to achieve the above objects, the present invention provides a throwaway insert comprising: an insert body formed in a polygonal shape; a cutting face formed by a polygonal face of the insert body; a corner cutting edge which is formed in a curved shape at a corner of the cutting face; a major cutting edge which is formed along a side edge of the cutting face that is connected to a first end of the corner cutting edge; and a concave portion which is formed, in another side edge of the cutting face that is connected to the other end of the corner cutting edge, so as to be adjacent to the other end of the corner cutting edge, and so as to be made concave toward the inside of the cutting face from a tangent of the corner cutting edge drawn at the other end of the corner cutting edge.
The present invention further provides a throwaway cutting tool comprising: a cutting tool main body which is rotatable about an axis of rotation and which has a distal end and a proximal end; and a throwaway insert, as a positive throwaway insert, of the present invention, wherein the throwaway insert is detachably attached to an outer periphery of the distal end of the cutting tool main body in such a manner that the cutting face thereof is directed in the direction of rotation of the cutting tool main body, that the corner cutting edge thereof is disposed at the distal end of the cutting tool main body while being projected radially and outwardly from the distal end, that the major cutting edge connected to a first end of the corner cutting edge is projected distally in the axial direction of the cutting tool main body while being inclined so as to depart from the distal end as the distance from the end of the corner cutting edge increases, and the distance from the center of the cutting tool main body decreases, and that a tangent drawn at the other end of the corner cutting edge is inclined toward the inside of the cutting tool main body as the distance from the proximal end of the cutting tool main body decreases.
In the above throwaway insert, a concave portion, which is shaped so as to be made concave from a tangent drawn at the second end of the corner cutting edge, is provided in the side edge that is connected to the second end of the corner cutting edge which is disposed so as to project radially and outwardly from the distal end of the cutting tool main body as viewed in a state in which the throwaway insert is attached to the cutting tool main body, and thus a larger gap between the surface (the wall) being machined and the throwaway insert can be ensured by the size of the concave portion which is made concave toward the inside of the cutting face when compared with the cases of conventional throwaway inserts and cutting tools. Moreover, another corner cutting edge may be provided at a portion of the cutting face diagonally opposing the above-mentioned corner cutting edge along with another concave portion adjacent to another corner cutting edge as in a conventional throwaway insert, or alternatively, the insert body may be formed as an equilateral polygon, and concave portions and adjacent concave portions may be provided corresponding to all corners of the polygon, respectively. As a result, another concave portion is formed in another side edge of the cutting face, and fixing rigidity of the insert may be increased to prevent shifting of the insert during a cutting operation if a convex portion, which is engageable with a recess in the insert body that is connected to another concave portion, is formed on the cutting tool main body.
In order to further ensure a large gap between the throwaway insert and the surface being machined, the concave portion may preferably be formed so as to be connected to the second end of the corner cutting edge, i.e., the concave portion preferably starts concaving toward the inside of the cutting face right at the second end of the corner cutting edge. In addition, if the bottom of the concave portion is formed as a straight line, the bottom surface of the recess connected to the concave portion is formed as a plane, and as a result, fixing rigidity of the insert may be further increased if a convex portion, which is engageable with a recess in the insert body that is connected to another concave portion, is formed on the cutting tool main body. On the other hand, if the bottom of the concave portion is formed as a curved concave line, a larger gap between the throwaway insert and the surface being machined may be ensured.
In the above throwaway insert, the major cutting edge may preferably be formed along a tangent of the corner cutting edge drawn at the second end of the corner cutting edge, or alternatively, the major cutting edge may preferably be formed by folded lines consisting of a tangent of the corner cutting edge drawn at the second end of the corner cutting edge and another line intersecting the tangent at a point opposite to the corner cutting edge at an obtuse angle. As a result, when the throwaway insert is attached to the cutting tool main body in such a manner that the major cutting edge is projected distally in the axial direction of the cutting tool main body so as to have a predetermined cutting edge angle while being inclined so as to depart from the distal end as the distance from the first end of the corner cutting edge increases, and the distance from the center of the cutting tool main body decreases as described above, the contact length between the major cutting edge and a workpiece being machined may be reduced, and thus the variation in cutting resistance can be restrained if compared with the case in which the entire major cutting edge is formed as an arc. In addition, if another concave portion, which is adjacent to another corner cutting edge, is further provided in a side edge opposing the corner cutting edge projecting radially and outwardly from the distal end of the cutting tool main body as described above, the flank, i.e., the side surface of the insert body, of another major cutting edge connected to an end of another corner cutting edge may be formed as a plane, and as a result, the support surface of the cutting tool main body can be reliably abutted against the flat flank, and the fixing rigidity of the throwaway insert may be further increased. In particular, if the major cutting edge is formed as two folded lines, an effect can be obtained that the cut chips made by the respective straight cutting edges are separated with respect to each other. In addition, because the cutting edge angle of a first straight portion of the major cutting edge, which is disposed, in an attached state, toward the proximal end of the cutting tool main body and outwardly with respect to the other (a second) straight portion while extending along the tangent drawn at the first end of the corner cutting edge, is greater than the cutting edge angle of the second straight portion of the major cutting edge, which is disposed inwardly with respect to the first straight portion, cutting is performed by the second straight portion when the depth of cut is relatively small, and on the other hand, when depth of cut is large, e.g., when a vertically extending wall is machined, the first straight portion also contributes to cutting, and as a result, an increase in cutting resistance may be restrained. If the major cutting edge comprises more than three straight portions, the variation in cutting resistance may be increased due to a large contact length between the major cutting edge and a workpiece, and it is difficult to manufacture such a major cutting edge; therefore, one straight portion or two straight portions are preferable for forming the major cutting edge.
In order to improve surface roughness of the plane portion being machined, the throwaway insert may preferably comprise a minor cutting edge which is provided on a side edge of the cutting face so as to be connected to an end of the major cutting edge opposite to the corner cutting edge, and which is shaped so as to gradually deviate from an extended line of the major cutting edge toward the inside of the cutting face. A smoother plane portion may be obtained by using the minor cutting edge while disposing the minor cutting edge substantially along the plane perpendicular to the axis of the cutting tool main body. If the minor cutting edge is formed in a circular arc shape, the surface roughness of the finished surface may be further improved while preventing a large step from being formed in the plane portion at every feed of the cutting edge even when the minor cutting edge is not disposed strictly along the plane perpendicular to the axis of the cutting tool main body. When the minor cutting edge is formed in a circular arc shape, the radius of the circular arc may be preferably set to be 1 to 100 times D, where D is the diameter of an inscribed circle of the cutting face. If the radius of the circular arc is set to be out of the range, the above-mentioned effects may not be obtained. The throwaway cutting tool may comprise a shaft-shaped shank extending along the axis of rotation on the rear end portion of the cutting tool main body. If the throwaway cutting tool is held in an attachment hole of a holder which can be attached to the major spindle of a machine tool, dimensions including the diameter of cutting edge circle of the tool may preferably be indicated on a tip surface of the cutting tool main body.